Structure functions at low x and Q 2 at ZEUS Jonathan Scott ZEUS Collaboration University of Bristol 8th International Workshop on Deep Inelastic Scattering and QCD Liverpool, April 2000
Feb 05, 2016
Structure functions atlow x and Q2 at ZEUSStructure functions atlow x and Q2 at ZEUS
Jonathan ScottZEUS Collaboration
University of Bristol
8th International Workshop onDeep Inelastic Scattering and QCD
Liverpool, April 2000
Introduction
HERA provides a unique opportunity to explorethe transition region between perturbative and
non perturbative QCD.
The kinematic region at low Q2 and x can be reached using small angle detectors and
Initial State radiative events.
This presentation• Kinematics • BPC/BPT Detectors• Results• Possibilities from ISR• Summary
Negative 4-momentum squared of virtual photon
Bjorken scaling
Fractional energy loss of electron in proton rest frame
Mass2 of Hadronic system
Kinematics 1
2sinE4E)k(kqQ 2
ee222
sy
Q
2p.q
Qx
22
)cos(12E
E-1y
e
e
27.5GeV positrons
820GeV protons
x
xQqPW
)1()(
222
Kinematics 2
Differential cross section for * + p X is
)(
23
224
2
2
2
xFYFyFYyQdxdQ
dL
2)1(1 yY
FL Longitudinal structure function - relevant at high y
xF3 - Parity violating Term
Low x approximation
),(4
),( 222
2**2* QxF
QQw p
Lp
Tp
tot
)(22 qqxeF
Quarksq
Kinematic Region
Lowest Q2 bin 0.045GeV2 Highest y bin 0.84Lowest y bin 0.005
BPC/BPT
BPC (Beam Pipe Calorimeter)Tungsten-scintillator sampling calorimterScattering angle range 18-32mradEnergy resolution 17%/EExtend Q2 coverage down to 0.1GeV2
BPT (Beam Pipe Tracker)2 Silicon Microstrip detectors
Improve Q2 resolutionVertex resolution 3cm
Control systematicsBgd rejection
BPC/BPT Bins
High y E´e > 4.4GeV Electron Method
Medium y E´e > 7GeV Electron Method
Low y(+prescale)E´e > 20GeV Sigma Method
BPC/BPT event selection
Electromagnetic shower in BPC
Track in BPT1. Matching with BPCReject fake positrons (Php +radiative)2. Originates from valid
interaction vertexReject beam related background
Background cuts on = (E-pZ)TotalyJB = (E-pZ)Hadronic/2Ee
3.9pb-1 data 5.3pb-1 non-diffractive MC 0.9pb-1diffractive MC
Control Plots
Control Plots
Results
Results
Results
ISRFL sensitive to QCD properties which
cannot be explored frommeasurements of F2.
FL measurement requiresmeasurement of differential cross-section at different centre of massenergies i.e different y for same
(x,Q2). Very difficult measurement.
Use Initial State Radiation toEffectively vary the beam energy.
FL
D i f f e r e n t i a l c r o s s s e c t i o n f o r * + p Xi n t e r m s o f R . .
24
2
2
2
1
12F
R
RY
xQdxdQ
d
W h e r e
L
LL
T
L
FF
F
QxxF
QxFQxR
22
1
22
),(2
),(),(
2)1(1
)1(2
y
y
2)1(1 yY
M e a s u r e I S R p h o t o n i n L U M I - c a l o r i m e t e r .
U n r e s o l v e d i s s u e sL U M I - a c c e p t a n c eL a r g e B a c k g r o u n d .
ISR Kinematic range
Bridges Q2 range between BPC/BPT and nominal F2 analyses as well
as overlap in y with Fixed Target experiments
Summary
T h e B P T w a s i n s t a l l e d i n 1 9 9 7 t o i m p r o v e t h e d e t e c t i o n o f p o s i t r o n s ,
i m p r o v i n g t h e q u a l i t y o f l o w Q 2 d a t a .
T h i s a l l o w e d t h e m e a s u r e m e n t o f F 2
a n d p
tot* i n t h e r e g i o n
0 . 0 4 5 G e V 2 < Q 2 < 0 . 6 5 G e V 2
6 . 1 0 - 7 < x < 1 . 1 0 - 3
I n i t i a l s t a t e r a d i a t i v e e v e n t s s h o u l d
p r o v i d e m e a s u r e m e n t o f F 2 i n t h e r e g i o n s p a n n i n g B P T d a t a a n d
n o m i n a l F 2 r e s u l t s , a s w e l l a s g i v i n g a f i r s t d i r e c t m e a s u r e m e n t o f F L .